1. Field of the Invention
The invention pertains to devices employing the parametric coupling of two signals and, particularly, to broadband parametric amplifiers.
2. Description of the Prior Art
Parametric amplifiers operate on the basis of periodically varying the capacitance of a varactor diode by means of a high-frequency pump signal and transferring energy to a lower frequency input signal by mixing of the input frequency with the pump frequency through an idler frequency. The Manley-Rowe relations show that current must flow at the three frequencies satisfying the relation: EQU .sup.F pump = .sup.F signal + .sup.F idler
for gain to exist at the signal frequency. Provisions must therefore be made to present a short circuit or very low impedance to the diode terminals at the idler frequency to allow current to flow at this frequency without introducing excessive losses. In addition, it is desirable that an open circuit appears to the diode terminals to prevent current flow at unwanted frequencies within the parametric amplifier. These frequencies include the second and higher harmonics of the pump, signal and idler frequencies and the upper sideband frequency which is the sum of the pump and input signal frequencies.
In the prior art, devices employing the parametric coupling of two signals to achieve a desired result have typically been suitable only for narrowband applications. Although some broadband devices have been developed, these devices made no attempt to limit the upper sideband signal which is complementary to the useful idler signal, and were therefore, noisy devices which were unsuitable for many applications. Devices which have attempted to limit noise by controlling the current at the upper sideband frequency, such as described in U.S. Pat. No. 3,609,571 of G. I. Klein and R. L. Zahm, have utilized tuning mechanisms, which when properly adjusted, substantially eliminated the upper sideband frequency. However, as these devices are tuned to eliminate the upper sideband frequency, the impedance at the pump signal is also affected which causes these devices to become less efficient requiring high pump power.
The present invention satisfies the need for high gain parametric devices which provide a low noise figure by inhibiting the current at the upper sideband frequency independently of the impedance established at the pump frequency so that the device's broadband performance and low power requirement are not compromised.